Martensitic Stainless Steel and Manufacturing Process Therefor

ABSTRACT

The present invention provides a martensitic stainless steel and manufacturing process therefor, the mass ratio of chemical elements of the stainless steel is C: 0.01˜0.18 wt %, Si: 0.4˜1.5 wt %; Mn: 0.4˜3.0 wt %, P: ≤0.04 wt %, S: 0.002˜0.01 wt %, Cr: 11.0˜15.0 wt %, N: 0.01˜0.15 wt %, Nb: 0.001˜0.01 wt %, V: 0.05˜0.25 wt %, Ti: 0.001˜0.01 wt %, Mo: 0.01˜1.50 wt %, B: 0.0005˜0.001 wt %, and balance being Fe and other unavoidable impurities. Further, the present invention provides a method for producing a martensitic stainless steel. The martensitic stainless steel of this present patent has a high hardness and high toughness and has excellent high-temperature oxidation resistance.

TECHNICAL FIELD

The present invention relates to a metal material and processing method therefor, in particular to a martensitic stainless steel and manufacturing process therefor.

BACKGROUND OF INVENTION

Martensitic stainless steel is a chromium-based stainless steel that is widely used in fields such as cutters, gauges, turbine blades, etc. where toughness and corrosion resistance are required. The low-carbon martensitic stainless steel having both a relatively high hardness (30˜40 HRC) and good toughness (Charpy V-notch impact energy is more than 30 J) are usually required by the users. For martensitic stainless steel, the strength and hardness after heat treatment are mainly increased by adding carbon element. However, increasing the carbon content lowers the toughness, so high strength and high toughness are always two properties that martensitic steel cannot have simultaneously. Since the user needs to subject the martensitic stainless steel to a heat treatment during use, high temperature heat treatment often causes a relatively thick oxide layer on the surface of the martensitic stainless steel, which brings great difficulties to the downstream user's surface polishing process, so the high temperature oxidation resistance is also an important indicator of the performance of the martensitic stainless steel.

In the prior art, the Chinese patent CN101906587A provides a low-carbon martensitic stainless steel for a brake disc, wherein the stainless steel having a high strength and toughness by using a low carbon content (0.03˜0.1 wt %) and a high manganese content (1˜2.5 wt %). However, the patent controls silicon as less than 0.5% by weight of an impurity element, thereby the high temperature oxidation resistance of the invention is relatively poor. Chinese patent CN103255340B proposes a high-strength hot-formed steel plate and a preparation method thereof to overcome the problem of high strength and insufficient toughness after the formation of the high-strength automobile steel, wherein the steel plate is heated to an austenitizing temperature at a heating speed of 20-100° C./s. The steel plate was kept at the austenitizing temperature for a period of time and subjected to a hot-rolling, and thereby the austenite grains were refined; and then the plate quenched to 50-370° C. at a speed of 50-120° C./s to obtain partially supersaturated martensite and untransformed residual austenite; and then the plate was maintained a tempering temperature of 200-500° C. for 5-600 s, to partition the carbon is from martensite to residual austenite to stabilize austenite; finally the plate was quenched to room temperature to obtain a complex phase structure of refined martensite and residual austenite.

Thus, high strength and high toughness steel can be obtained. Such method of quenching and partitioning to arrive at a complex phase structure and a combination of high strength and high toughness has been widely used in carbon steels. For example, a formulation of steel is proposed in CN103160680A, wherein a complex phase structure of refined martensite and residual austenite was obtained using a combinative quenching technique. The steel had a strength-ductility product index of more than 30 GP wt %; CN103243275B proposes a low-alloy high-strength steel, wherein a complex phase structure of bainite, martensite and austenite was obtained through the partitioning and tempering treatment, having a good combination of strength and ductility; CN103045950B also proposes a low-alloying low-cost steel, wherein the strength of the steel was increased and strength was maintained through rapid quenching and carbon repartitioning. Quenching and partitioning method is not often used in stainless steel. In the prior art, CN103614649B proposes a martensitic stainless steel comprising a carbon content of 0.15-0.4 wt %, a nitrogen content of 0-0.12 wt % and a chromium content of 13.0-17.0 wt %, a Ni content of 0-5%, a molybdenum content of 0-2.0 wt %, wherein the stainless steel was made as follows: a hot-rolled slab made from conventional materials was heated to 950-1100° C. and insulated for 0.5-2 h, and then air-cooled to 25-200° C., then heated to 350-500° C. and insulated for 10-60 min, air-cooled to room temperature. In the process, through quenching and partitioning method was used to disperse the residual austenite into the microstructure, and thereby significantly increased the strength and ductility of the martensitic stainless.

In summary, there are problems that the high hardness and the high toughness of the martensitic stainless steel, as well as the high-temperature oxidation resistance cannot be combined in the prior art.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, the first object of the present invention is to provide a martensitic stainless steel having a specific composition. The martensitic stainless steel has a high hardness and high toughness and has excellent high-temperature oxidation resistance.

In order to achieve the above object, the present invention provides a martensitic stainless steel, the mass ratio of chemical elements is C: 0.01˜0.18 wt %, Si: 0.4˜1.5 wt %, Mn: 0.4˜3.0 wt %, P: ≤0.04 wt %, S: 0.002˜0.01 wt %, Cr: 11.0˜15.0 wt %, N: 0.01˜0.15 wt %, Nb: 0.001˜0.01 wt %, V: 0.05˜0.25 wt %, Ti: 0.001˜0.01 wt %, Mo: 0.01˜1.50 wt %, B: 0.0005˜0.001 wt %, and balance being Fe and other unavoidable impurities.

Optionally, the mass ratio of chemical elements is C: 0.03˜0.15 wt %, Si: 0.53˜1.25 wt %, Mn: 0.45˜2.45 wt %, Cr: 11.2˜14.5 wt %, N: 0.01˜0.08 wt %, Nb: 0.001˜0.007 wt %, V: 0.05˜0.23 wt %, Ti: 0.002˜0.008 wt %, Mo: 0.01˜1.10 wt %, B: 0.0005˜0.0009 wt %, and balance being Fe and other unavoidable impurities.

Optionally, the mass ratio of chemical elements satisfies a relationship: 0.02 wt %≤C+N≤0.20 wt %.

Optionally, the mass ratio of chemical elements satisfies a relationship: V+Ti+Nb≤0.25 wt %.

Optionally, the microstructure of the martensitic stainless steel is a complex phase structure of martensite and residual austenite.

Optionally, in the microstructure, the phase ratio of the martensite is 55˜65%, and the phase ratio of the residual austenite is 35˜45%.

Optionally, the martensitic stainless steel is prepared by the following steps:

(1) Hot rolling of steel billets or continuous casting billets into hot rolled steel plates or hot rolled steel strips, and annealing;

(2) Heating the annealed hot-rolled steel plates or hot rolled steel strips to 850˜1000° C. and insulating for 5˜30 min, then rapidly cooling to a two-phase zone of martensite and austenite with a speed greater than or equal to 30° C./s. The cooling termination temperature is 150˜220° C. Then, heating the hot-rolled steel plates or hot rolled steel strips to 350˜500° C., and air-cooling to room temperature to obtain a martensitic stainless steel.

The present invention provides a martensitic stainless steel, being an alloy system of C—Si—Mn—Cr—N—Nb—V—Ti—Mo—B. The chemical composition of the stainless steel makes the stainless steel have a high hardness, high toughness and excellent oxidation resistance. The rationale for the specific chemical composition is as follows:

C: it is an important austenitizing element. Certain carbon content ensures that a complete austenite structure can be obtained at high temperature. It is also an important element for ensuring the hardness after heat treatment. Carbon is an important solid solution strengthening element and precipitation strengthening element. It can exist in a form of interstitial atoms in the steel. During the reheating process after quenching, repartitioning of carbon can be accomplished by interphase diffusion to stabilize the residual austenite structure.

However, excessive carbon content increases brittleness on the one hand, and degrades corrosion resistance on the other hand. In order to achieve the desired effect, the carbon in the present invention is used in combination with nitrogen element and having a content of 0.01˜0.18 wt %, and preferably 0.03˜0.15 wt %.

N: Similarly with carbon, it is an austenitizing element and can exist in the form of interstitial atoms. It has a solid-solution strengthening effect. Nitrogen has a higher solubility in austenite than carbon, and there are fewer nitrogen precipitates during heat treatment. Additional, nitrogen dissolved in the matrix can improve the corrosion resistance of stainless steel. Therefore, nitrogen is an element that can not only improve the strength of martensitic stainless steel, but also improve the corrosion resistance. In the present invention, the content of N is 0.01 to 0.15 wt %, and preferably 0.01 to 0.08 wt %.

Si: mainly added to steel as a deoxidizer, it plays a role of solid solution strengthening, and it also has a significant role in improving high temperature oxidation resistance. However, a high silicon content may decrease the ductility of the steel. Therefore, from the viewpoint of improving the oxidation resistance of the stainless steel without degrading the processability, the content thereof is 0.4˜1.5 wt %, preferably 0.53˜1.25 wt % in the present invention.

Mn: manganese is both a deoxidizing element and a solid solution strengthening element, which can significantly increase the strength of steel. In addition, since manganese is an austenitizing element, the addition of manganese can make it easier for martensitic stainless steel to form austenite at high temperature, thereby obtaining more martensite upon cooling. However, too high manganese content is not conducive to annealing softening, and the content thereof in the present invention is 0.4˜3.0 wt %, preferably 0.45˜2.45 wt %.

P: phosphorus is a harmful element, and therefore the content of is reduced as much as possible according to the level of production control.

S: sulfur is also a harmful element. The resulting sulfides not only produce hot brittleness (When the steel is hot processed at 1100-1200° C., the low-melting eutectic distributed in the grain boundary melts and causes cracking. This is so-called “hot brittleness” phenomenon of sulfur), but also reduces the corrosion resistance. Usually the sulfur content is controlled below 0.01% by weight to avoid the harmful effects of sulfur.

Cr: it is an element that improves the corrosion resistance of stainless steel. However, chromium is a strong ferrite forming element. When the content of Cr is high, the austenitizing of low-carbon martensitic steel becomes difficult, and the cost may increase. In the present invention, the chromium content is 11.0˜15.0 wt %, preferably 11.2˜14.5 wt %.

In order to obtain a austenite with high hardness (30˜50 HRC) and high toughness, for the combined use of carbon and nitrogen, 0.02 wt %≤C+N≤0.15 wt % is required. A high total content of C and N may result in a too high hardness of the material and deteriorate the toughness.

Mo: molybdenum improves hardenability and thermal strength in steel. It can prevent quenching brittleness, and effectively improve the corrosion resistance of martensitic stainless steel in the general medium of air or water, but the increase of Mo also increases the precipitation of FeCrMo phase in stainless steel, which affects the toughness and corrosion resistance of stainless steel. Therefore, the content of Mo is strictly limited. In the present invention, the Mo content is 0.01˜1.50 wt %, preferably 0.01˜1.10 wt %.

V, Ti, Nb: all are strong carbide elements, and they are very easy to react with the interstitial atoms and form carbon and nitrides during thermal processing or heat treatment, and thus make the interstitial atoms lose the ability of partitioning between phases, so the content of vanadium, titanium and niobium is preferably controlled to be V+Ti+Nb≤0.25 wt % in the present invention.

B: in the process of austenite transformation, the ferrite of boron is most likely to be nucleated at the grain boundary. Because B is adsorbed on the grain boundary, it fills in defects and reduces the grain boundary energy level, making the new phase difficult to nucleate. Therefore, the austenite has an increased stability, and thus an improved hardenability. In the present invention, the content thereof is 0.0005˜0.001 wt %, and preferably 0.0005˜0.0009 wt %.

Among the strengthening alloy elements of martensitic stainless steels, carbon and nitrogen are the most effective elements for increasing strength. The addition of carbon tends to cause carbon segregation during rolling and heat treatment and reduces the corrosion resistance of martensitic stainless steels. And the addition of nitrogen can easily cause a large number of pores in the stainless steel substrate, which affects the forming and polishing. Therefore, in the present invention, the content of carbon and nitrogen elements are reasonably controlled and the steel-making impurity elements such as Si, Mn, and the likes are reasonably used, the alloying elements such as Mo, V, Nb, Ti, B, and the likes are added and mixed, and accordingly the low-carbon martensitic stainless steel can satisfy the index of high carbon martensitic stainless steel in terms of strength and hardness, while maintain the characteristics of low carbon martensitic stainless steel in the term of toughness and corrosion resistance, and thus effectively solve the problem of the incompatible and mismatching properties, e.g. strength, toughness, corrosion resistance and etc. of the martensitic stainless steel.

The second object of the present invention is to provide a method for producing a martensitic stainless steel, by which a martensitic stainless steel having high hardness, high toughness, and excellent high-temperature oxidation resistance can be finally produced.

The present invention provides a method for producing a martensitic stainless steel, including the following steps:

(1) Hot rolling of steel billets or continuous casting billets into hot rolled steel plates or hot-rolled steel strips, and annealing;

(2) Heating the annealed hot-rolled steel plates or hot rolled steel strips to 850˜1000° C. and insulating for 5˜30 min, then rapidly cooling to a two-phase zone of martensite and austenite with a speed greater than or equal to 30° C./s. The cooling termination temperature is 150˜220° C. Then, heating the hot-rolled steel plates or hot-rolled steel strips to 350˜500° C., and air-cooling to room temperature to obtain a martensitic stainless steel;

wherein the mass ratio of chemical elements of the martensitic stainless steel is C: 0.01˜0.18 wt %, Si: 0.4˜1.5 wt %, Mn: 0.4˜3.0 wt %, P: ≤0.04 wt %, S: 0.002˜0.01 wt %, Cr: 11.0˜15.0 wt %, N: 0.01˜0.15 wt %, Nb: 0.001˜0.01 wt %, V: 0.05˜0.25 wt %, Ti: 0.001˜0.01 wt %, Mo: 0.01˜1.50 wt %, B: 0.0005˜0.001 wt %, and balance being Fe and other unavoidable impurities.

Optionally, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.03˜0.15 wt %, Si: 0.53˜1.25 wt %, Mn: 0.45˜2.45 wt %, Cr: 11.2˜14.5 wt %, N: 0.01˜0.08 wt %, Nb: 0.001˜0.007 wt %, V: 0.05˜0.23 wt %, Ti: 0.002˜0.008 wt %, Mo: 0.01˜1.10 wt %, B: 0.0005˜0.0009 wt %, and balance being Fe and other unavoidable impurities.

Optionally, the mass ratio of chemical elements of the martensitic stainless steel satisfies a relationship: 0.02 wt %≤C+N≤0.20 wt %.

Optionally, the mass ratio of chemical elements of the martensitic stainless steel satisfies a relationship: V+Ti+Nb≤0.25 wt %.

Optionally, the microstructure of the martensitic stainless steel is a complex phase structure of martensite and residual austenite.

Optionally, in the microstructure, the phase ratio of the martensite is 55˜65%, and the phase ratio of the austenite is 35˜45%.

Hot rolling of the steel billets or continuous casting billets with determined composition into hot rolled steel plates or hot rolled steel strips, and annealing; and the annealed plates or strips has a microstructure of ferrite and carbide, a relatively low hardness and high ductility, and is suitable for punching, shearing, and rolling processing.

Heating the annealed hot-rolled steel strips to 850˜1000° C. and insulating for 5˜30 min, mainly to ensure that the steel can be fully austenitized, and carbon and nitride can be fully solutionize.

Then rapidly cooling the plates or strips to a two-phase zone of martensite and austenite with a speed greater than or equal to 30° C./s, i.e., the temperature is cooled to between the start temperature (Ms) and the final temperature (Mf) of the martensite transformation to obtain the duplex structure of martensite and austenite. The Ms is calculated as Ms(° C.)=539−430×[C+N]−30×[Mn]−12×[Cr]−5.0×[Si], among them, [C], [Si], [Mn], [Cr], [N] are the weight contents of C, Si, Mn, Cr, and N in the martensitic stainless steel respectively; the Mf is calculated as Mf(° C.)=Ms-250. Rapid cooling at a rate of greater than or equal to 30° C./s can avoid the precipitation of carbide and nitride during cooling. The introduction of dispersed residual austenite in the microstructure by means of quenching and partitioning significantly increases the strength and ductility of the martensitic stainless steel.

Reheating to 350˜500° C. and insulating for 10˜30 min, and allows the carbon and nitrogen interstitial atoms diffuse into the austenite from the martensite structure, which increases the stability of the austenite structure in which the martensite transformation has not been completed, and thereby increases the stability of stainless.

Air-cooling to room temperature to obtain the martensitic stainless steel, which having high hardness and high toughness.

The present invention provides a method for producing a martensitic stainless steel, the martensitic stainless steel produced by the method has a complex phase structure of martensite and residual austenite. The stainless steel also has high hardness, high toughness and excellent high-temperature oxidation resistance.

DETAILED DESCRIPTION OF THE INVENTION

The following are specific embodiments of the implementation of the present invention, those skilled in the art can easily understand other advantage and effects of the present invention from the contents disclosed in this specification. Although the present invention will be described in combination with the preferred embodiment, it is understood that the features of this invention are not limited to the preferred embodiment. On the contrary, the purpose of presenting the present invention in combination with the preferred embodiments is to cover other alternatives or modifications that may be derived from the claims of the present invention. The following description will include abundant specific details to facilitate a deeper understanding of the present invention. The present invention may also be implemented without using these details. In addition, some specific details will be omitted in the description so as to avoid confusion and missing the key points of the present invention.

The difference between examples 1-5 and comparative examples 1-3 is that the chemical composition ratio and the process parameters are different. See Table 1 and Table 2 for details.

Example 1

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled strips were heated to 960° C. and insulated for 20 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 200° C. Then the strips were reheated to 350° C., and insulated for 20 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found for example 1 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.05 wt %, Si: 0.53 wt %, Mn: 2.45 wt %, P: 0.02 wt %, S: 0.004 wt %, Cr: 12.3 wt %, N: 0.05 wt %, Nb: 0.001 wt %, V: 0.05 wt %, Ti: 0.003 wt %, Mo: 0.01 wt %, B: 0.0005 wt %, and balance being Fe and other unavoidable impurities.

Example 2

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled strips were heated to 920° C. and insulated for 10 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 175° C. Then the strips were reheated to 400° C. and insulated for 10 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found for example 2 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.08 wt %, Si: 0.63 wt %, Mn: 1.34 wt %, P: 0.02 wt %, S: 0.006 wt %, Cr: 11.2 wt %, N: 0.07 wt %, Nb: 0.002 wt %, V: 0.07 wt %, Ti: 0.002 wt %, Mo: 0.01 wt %, B: 0.0006 wt %, and balance being Fe and other unavoidable impurities.

Example 3

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled steel strips were heated to 920° C. and insulated for 5 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 165° C. Then the strips were reheated to 400° C. and insulated for 10 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found for example 3 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.07 wt %, Si: 0.87 wt %, Mn: 1.78 wt %, P: 0.03 wt %, S: 0.002 wt %, Cr: 13.8 wt %, N: 0.08 wt %, Nb: 0.005 wt %, V: 0.08 wt %, Ti: 0.005 wt %, Mo: 0.5 wt %, B: 0.0007 wt %, and balance being Fe and other unavoidable impurities.

Example 4

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled steel strips were heated to 880° C. and insulated for 30 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 165° C. Then the strips were reheated to 450° C. and insulated for 30 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found in example 4 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.03 wt %, Si: 1.25 wt %, Mn: 0.45 wt %, P: 0.02 wt %, S: 0.001 wt %, Cr: 14.5. wt %, N: 0.01 wt %, Nb: 0.006 wt %, V: 0.15 wt %, Ti: 0.008 wt %, Mo: 1.1 wt %, B: 0.0008 wt %, and balance being Fe and other unavoidable impurities.

Example 5

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled steel strips were heated to 880° C. and insulated for 20 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 165° C. Then the strips were reheated to 500° C. and insulated for 20 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found in example 5 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.03 wt %, Si: 1.25 wt %, Mn: 0.45 wt %, P: 0.02 wt %, S: 0.008 wt %, Cr: 14.5 wt %, N: 0.01 wt %, Nb: 0.006 wt %, V: 0.15 wt %, Ti: 0.008 wt %, Mo: 1.1 wt %, B: 0.0008 wt %, and balance being Fe and other unavoidable impurities.

Comparative Example 1

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled steel strips were heated to 920° C. and insulated for 20 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 25° C. Then the plates or strips was reheated to 250° C. and insulated for 30 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found in Comparative Example 1 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.13 wt %, Si: 1.42 wt %, Mn: 0.35 wt %, P: 0.03 wt %, S: 0.001 wt %, Cr: 11.5 wt %, N: 0.03 wt %, Nb: 0.3 wt %, V: 0.01 wt %, Ti: 0.3 wt %, Mo: 0 wt %, B: 0.01 wt %, and balance being Fe and other unavoidable impurities.

Comparative Example 2

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled steel strips were heated to 880° C. and insulated for 20 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 165° C. Then the plates or strips was reheated to 500° C. and insulated for 20 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found in Comparative Example 2 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.15 wt %, Si: 1.5 wt %, Mn: 0.56 wt %, P: 0.04 wt %, S: 0.009 wt %, Cr: 12.6 wt %, N: 0.04 wt %, Nb: 0 wt %, V: 0 wt %, Ti: 0 wt %, Mo: 1.1 wt %, B: 0 wt %, and balance being Fe and other unavoidable impurities.

Comparative Example 3

The steel billets or continuous casting billets were hot rolled into hot rolled steel plates or hot rolled steel strips, and annealed; the annealed hot-rolled steel strips were heated to 980° C. and insulated for 15 min, then rapidly cooled to a two-phase zone of martensite and austenite in a wind-cooling manner. The cooling termination temperature was 25° C. Then the strips were reheated to 250° C. and insulated for 20 min, and air-cooled to room temperature to obtain the martensitic stainless steel. Specific performance parameters can be found in comparative example 3 in Table 2. As shown in Table 1, the mass ratio of chemical elements of the martensitic stainless steel is C: 0.05 wt %, Si: 0.35 wt %, Mn: 2.0 wt %, P: 0.03 wt %, S: 0.005 wt %, Cr: 13 wt %, N: 0.04 wt %, Nb: 0.005 wt %, V: 0.1 wt %, Ti: 0.005 wt %, Mo: 0.7 wt %, B: 0.0008 wt %, and balance being Fe and other unavoidable impurities.

TABLE 1 Chemical compositions of the examples and comparative examples of the present invention (wt %) No. C Si Mn P S Cr N V Nb Ti Mo C + N B V + Ti + Nb Example 1 0.05 0.53 2.45 0.02 0.004 12.3 0.05 0.05 0.001 0.003 0.01 0.10 0.0005 0.054 Example 2 0.08 0.63 1.34 0.02 0.006 11.2 0.07 0.07 0.002 0.002 0.01 0.15 0.0006 0.074 Example 3 0.07 0.87 1.78 0.03 0.002 13.8 0.08 0.08 0.005 0.005 0.5 0.15 0.0007 0.09 Example 4 0.03 1.25 0.45 0.02 0.001 14.5 0.01 0.15 0.006 0.008 1.1 0.04 0.0008 0.164 Example 5 0.15 1.12 0.56 0.02 0.008 12.6 0.01 0.23 0.007 0.006 1.5 0.02 0.0009 0.243 Comparative 0.13 1.42 0.35 0.03 0.001 11.5 0.03 0.01 0.3 0.3 0 0.16 0.01 0.61 example 1 Comparative 0.15 1.5 0.56 0.04 0.009 12.6 0.04 0 0 0 0 0.19 0 ≤0.01 example 2 Comparative 0.05 0.34 2.0 0.03 0.005 13 0.04 0.1 0.005 0.005 0.7 0.045 0.0008 0.11 example 3 (The unit of each component is wt %, and balance being Fe and other unavoidable impurities.)

TABLE 2 Heat treatment process and performance of the examples and comparative examples of the present invention In- Cooling Gain in weight Heating sulating termination Reheating Insulating Rockwell at 1000° C. temperature/ time/ Cooling temperature/ temperature/ time/ Cooling hardness Impact for 100 hours No. ° C. min manner ° C. ° C. min manner HRC energy/J oxidation/mg/cm² Example 1 960 20 Wind 200 350 20 Air 30 38 2.0 cooling cooling Example 2 920 10 Wind 175 400 10 Air 33 35 2.1 cooling cooling Example 3 920 5 Wind 165 400 10 Air 35 33 1.9 cooling cooling Example 4 880 30 Wind 165 450 30 Air 39 33 2.8 cooling cooling Example 5 880 20 Wind 165 500 20 Air 38 31 3.0 cooling cooling Comparative 920 20 Wind 25 250 30 Air 40 13 13 example 1 cooling cooling Comparative 830 20 Wind 165 500 20 Air 43 26 15 example 2 cooling cooling Comparative 980 15 Wind 25 250 20 Air 28 22 4.0 example 3 cooling cooling

In the present invention, the ratio of martensite and austenite in the martensitic stainless steel was respectively 55˜65%, and 35˜45% determined by X-ray diffraction quantitative phase analysis.

Steel billets or continuous casting billets having the chemical composition shown in Table 1 were hot rolled into hot rolled steel plates or steel strips and annealed; then processed according to the heat treatment process shown in Table 2 to obtain steel plates with martensite and residual austenite complex phase; finally, performance tests were carried out. As shown in Table 2, the steel plates provided by the present invention has a Rockwell hardness of 30˜50 HRC, a Charpy V type notched impact energy of more than 30 J, and an oxidation gain in weight of less than 3.5 mg/cm² at 1000° C. for 100 hours. The larger of the Charpy V type notched impact energy (J), the better of the toughness of the steel plates, and less sensitive to the notch or other stress concentration in the steel plates; The value of gain in weight at 1000° C. for 100 hours oxidation (mg/cm²) indicates the high-temperature oxidation resistance of the steel plates, the smaller of the weight gain, the better of the high-temperature oxidation resistance of the steel plates.

In the embodiment of the present invention, the temperature of complex phase structure of martensite and residual austenite is between the start temperature (Ms) and the final temperature (Mf) of the martensite transformation. The Ms is calculated as Ms(° C.)=539−430×[C+N]−30×[Mn]−12×[Cr]−5.0×[Si], among them, [C], [Si], [Mn], [Cr], [N] are the weight contents of C, Si, Mn, Cr, and N in the martensitic stainless steel respectively; the Mf is calculated as Mf(° C.)=Ms-250. In the examples of the present invention, the temperature of complex phase structure of martensite and residual austenite, i.e., the cooling termination temperature was controlled at 150˜220° C. (falling within the range between Ms to Mf). As shown in Table 1, the cooling termination temperature of example 1 was 200° C., and the corresponding Ms (° C.) was 272.25 and Mf(° C.) was 22.25, which satisfied that the temperature of complex phase structure of martensite and residual austenite falling within the range between the start temperature (Ms) and the final temperature (Mf) of the martensite transformation. Similarly, the cooling termination temperature in examples 2-5 all satisfied the corresponding temperature range (the corresponding Ms-Mf temperature range).

According to the above calculation method, Ms (° C.) in comparative example 1 was 305.6 and Mf (° C.) was 55.6, and the corresponding cooling termination temperature was 25° C., falling without the range between 55.6 to 305.6° C.; the cooling termination temperature in comparative example 2 was 160° C., and satisfying the corresponding Ms-Mf temperature range (31.8-281.8° C.). In comparative example 3, the cooling termination temperature was 25° C., and did not satisfy the corresponding Ms-Mf temperature range (51.45-301.45° C.).

For comparative example 1, when the cooling termination temperature (25° C.) was less than the final temperature of martensite transformation Mf (55.6° C.), the martensitic transformation process was prolonged, and the microstructure in the steel strips was increasingly transformed into martensite, then the residual austenite in the strips was reduced accordingly. However, the martensite plays an important role in the strengthening of the steel, while austenite increases the toughness or plasticity. Furthermore, the reheating temperature of Comparative Example 1 is 250° C. (less than the reheating temperature in the heat treatment process of the present invention of 350 to 500° C.), which affects the stability of the residual austenite. It can be known from the chemical composition of Comparative Example 1 in Table 1 that the sum of the weight contents of vanadium, titanium, and tantalum elements was greater than 0.25 wt %.

Therefore, in the corresponding steel billets or continuous casting billets more carbides and nitrides were formed with the interstitial elements in the heat treatment process, thereby the diffusion of carbon and nitrogen interstitial atoms from the martensite structure into the austenite structure was affected i.e., the stability of austenite was affected. Meanwhile, as an austenitizing element, the addition of manganese can make austenite easier to be formed in the martensitic stainless steel at high temperature. The content of Mn element was 0.35 wt % (less than the content of Mn element of 0.4˜3.0 wt % defined in the present invention), also affected the stability of austenite. So, it can be seen from the performance parameters of the comparative example 1 in Table 2 that the final martensitic stainless steel had a high Rockwell hardness (40 HRC), but a poor toughness (the impact energy is 13 J, which is lower than the average value of the impact work of the present invention, 34 J), and martensitic stainless steels having both high hardness and high toughness were not obtained.

Similarly, the contents of the chemical components in Comparative Examples 2 and 3 were not within the scope of the martensitic stainless steel provided by the present invention, and the manufacturing methods thereof did not satisfy the heat treatment process of the present invention (see comparative examples 2 and 3 of Table 2). The final martensitic stainless steels had low Rockwell hardness, poor toughness and poor high-temperature oxidation resistance (see Table 2 comparative examples 2 and 3). In a case that both conditions are not within the range of the martensitic stainless steel chemical composition content and the process parameters of heat treatment according to the present invention, the final martensitic stainless steels can have a poor high-temperature oxidation resistance and fail to have both high hardness and high toughness.

In summary, according to the present invention provides martensitic stainless steel and manufacturing process therefor, a martensitic stainless steel having high hardness and high toughness and excellent high-temperature oxidation resistance can be provided, and is suitable for a brake disc.

The above-described embodiments merely illustrate the principles of the present invention and its effects, rather than limiting the invention. Any person skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by a person of ordinary skill in the art without departing from the spirit and technical thought disclosed in the present invention should still be covered by the claims of the present invention. 

1. A martensitic stainless steel, wherein the mass ratio of chemical elements is C: 0.01˜0.18 wt %, Si: 0.4˜1.5 wt %, Mn: 0.4˜3.0 wt %, P: ≤0.04 wt %, S: 0.002˜0.01 wt %, Cr: 11.0˜15.0 wt %, N: 0.01˜0.15 wt %, Nb: 0.001˜0.01 wt %, V: 0.05˜0.25 wt %, Ti: 0.001˜0.01 wt %, Mo: 0.01˜1.50 wt %, B: 0.0005˜0.001 wt %, and balance being Fe and other unavoidable impurities.
 2. The martensitic stainless steel according to claim 1, wherein the mass ratio of chemical elements is C: 0.03˜0.15 wt %, Si: 0.53˜1.25 wt %, Mn: 0.45˜2.45 wt %, Cr: 11.2˜14.5 wt %, N: 0.01˜0.08 wt %, Nb: 0.001˜0.007 wt %, V: 0.05˜0.23 wt %, Ti: 0.002˜0.008 wt %, Mo: 0.01˜1.10 wt %, B: 0.0005˜0.0009 wt %, and balance being Fe and other unavoidable impurities.
 3. The martensitic stainless steel according to claim 1, wherein the mass ratio of chemical elements satisfies a relationship: 0.02 wt %≤C+N≤0.20 wt %.
 4. The martensitic stainless steel according to claim 1, wherein the mass ratio of chemical elements satisfies a relationship: V+Ti+Nb≤0.25 wt %.
 5. The martensitic stainless steel according to claim 1, wherein the microstructure of the martensitic stainless steel is a complex phase structure of martensite and residual austenite.
 6. The martensitic stainless steel according to claim 5, wherein in the microstructure, the phase ratio of the martensite is 55˜65%, and the phase ratio of the residual austenite is 35˜45%.
 7. The martensitic stainless steel according to claim 1, wherein the martensitic stainless steel is prepared by the following steps: (1) Hot rolling of steel billets or contiguous casting billets into hot rolled steel plates or steel strips, and annealing; (2) Heating the annealed hot-rolled steel plates or steel strips to 850˜1000° C. and insulating for 5˜30 min, then rapidly cooling to a two-phase zone of the martensite and austenite with a speed greater than or equal to 30° C./s; the cooling termination temperature is 150˜220° C.; then heating the steel plates or strips to 350˜500° C., and air cooling to room temperature to obtain the martensitic stainless steel.
 8. A method for producing the martensitic stainless steel, which includes the following steps: (1) Hot rolling of steel billets or casting billets into hot rolled steel plates or steel strips, and annealing; (2) Heating the annealed hot-rolled steel plates or steel strips to 850˜1000° C. and insulating for 5˜30 min, then rapidly cooling to a two-phase zone of martensite and austenite with a speed greater than or equal to 30° C./s; the cooling termination temperature is 150˜220° C.; then heating the steel plates or strips to 350˜500° C., and air cooling to room temperature to obtain the martensitic stainless steel, wherein the mass ratio of chemical elements of the martensitic stainless steel is C: 0.01˜0.18 wt %, Si: 0.4˜1.5 wt %, Mn: 0.4˜3.0 wt %, P: ≤0.04 wt %, S: 0.002˜0.01 wt %, Cr: 11.0˜15.0 wt %, N: 0.01˜0.15 wt %, Nb: 0.001˜0.01 wt %, V: 0.05˜0.25 wt %, Ti: 0.001˜0.01 wt %, Mo: 0.01˜1.50 wt %, B: 0.0005˜0.001 wt %, and balance being Fe and other unavoidable impurities.
 9. The method according to claim 8, wherein the mass ratio of chemical elements of the martensitic stainless steel is C: 0.03˜0.15 wt %, Si: 0.53˜1.25 wt %, Mn: 0.45˜2.45 wt %, Cr: 11.2˜14.5 wt %, N: 0.01˜0.08 wt %, Nb: 0.001˜0.007 wt %, V: 0.05˜0.23 wt %, Ti: 0.002˜0.008 wt %, Mo: 0.01˜1.10 wt %, B: 0.0005˜0.0009 wt %, and balance being Fe and other unavoidable impurities.
 10. The method according to claim 8, wherein the mass ratio of chemical elements of the martensitic stainless steel satisfies a relationship: 0.02 wt %≤C+N≤0.20 wt %.
 11. The method according to claim 8, wherein the mass ratio of chemical elements of the martensitic stainless steel satisfies a relationship: V+Ti+Nb≤0.25 wt %.
 12. The method according to claim 8, wherein the microstructure of the martensitic stainless steel is a complex phase structure of martensite and residual austenite.
 13. The method according to claim 12, wherein in the microstructure, the phase ratio of the martensite is 55˜65%, and the phase ratio of the residual austenite is 35˜45%.
 14. The martensitic stainless steel according to claim 2, wherein the mass ratio of chemical elements satisfies a relationship: 0.02 wt %≤C+N≤0.20 wt %.
 15. The martensitic stainless steel according to claim 2, wherein the mass ratio of chemical elements satisfies a relationship: V+Ti+Nb≤0.25 wt %.
 16. The martensitic stainless steel according to claim 2, wherein the microstructure of the martensitic stainless steel is a complex phase structure of martensite and residual austenite.
 17. The method according to claim 9, wherein the mass ratio of chemical elements of the martensitic stainless steel satisfies a relationship: 0.02 wt %≤C+N≤0.20 wt %.
 18. The method according to claim 9, wherein the mass ratio of chemical elements of the martensitic stainless steel satisfies a relationship: V+Ti+Nb≤0.25 wt %.
 19. The method according to claim 9, wherein the microstructure of the martensitic stainless steel is a complex phase structure of martensite and residual austenite. 